Process and apparatus for obtaining smooth surfaces on films



March 14, LAMBERT ETAL PROCESS AND APPARATUS FOR OBTAINING SMOOTH SURFACES ON FILMS Filed Dec. 30, 1958 PROCESS AND APPARATUS FOR OBTAINING SMOOTH SURFACES ON FILMS Robert L. Lambert, Emporium, and William R. McKeirnan, Crosby, Pa., assignors, by mesne assignments, to Sylvania Electric Products Inc., Wilmington, DeL, a corporation of Delaware Filed Dec. 30, 1958, Ser. No. 785,787 8 Claims. (Cl. 18-15) This invention relates to a film forming method and apparatus particularly as applied to the manufacture of coating material to be applied to cathode bases in electron tubes to form the emissive material'on the cathode base.

For use as an emissive coating, the film is made of the triple carbonates of calcium, barium and strontium in a cellulosic binder and after film formation is applied as a coating to the cathode base, as a cathode sleeve, so as to provide a coating thereon of uniformly even thinness. In electron tubes of the grid controlled type, in order to increase the magnification factor of the tube it is often necessary to exceedingly closely space the grid and cathode, and this must be done without shorting the grid to the cathode. to be sure that the film is prepared of uniform thinness throughout and without protrusions or excrescences which would cause a thickening of the film in isolated areas.

It is mainly to achieve the smoothness and eveness of the film on both surfaces of the film that this invention is directed. Other objects of the invention will be made clear in the following specification when taken in con-v junction with the accompanying drawing in which:

Fig. 1 is a schematic presentation of a film-forming system embodying the invention and Fig. 2 is a section on the line 2-2 of Fig. 1.

Referring to the drawings with greater particularity,

at 10, Fig. 1, is disclosed ametallic hopper containing the triple carbonates suspended in a volatile cellulosic solution, for example, ethylcellulose dissolved in a mixture of toluene, ethanol and pentacetate all as is set Therefore, it is necessary forth more in detail in the application of Kerstctter and Wennin Serial No. 453,235, filed August 31, 1954, and assigned to the assignee of this application. Thehopper is of course open at its bottom and is air tight "at the top except for a valve 12 which may be opened to vent the system. A piping 14-, 15, conveys the suspension to the hopper from a mixing tank 16 provided with a motor driven stirrer 13 to maintain the mixture homogeneous, and the surface of the mixture is maintained under air pressure, as through a constant pressure air regulator valve 20. The pipe 15 has its inlet opening below the surface of the mixture in the mixing tank. The mixture in the hopper is fed onto a casting wheel 22 in thejform of a narrow cylinder mounted on a horizontal axle 24 driven in any well knownmanner at a rate to give a peripheral speed of about one and one half feet per second. The surface of the wheel is of highly polished nickel steel and is maintained scrupulously clean by a water soaked sponge 26 with water fed thereto from a water pipe 27, and rotating in an opposite sense to the rotationo-f the wheel in order to-sponge off any dust or 2,974,364 Patented Mar. 14, 1961 ice dirt on the surface of the wheel; a drain 28 beneath the sponge catches the excess water which is led off through a suitable drain hose 29. The apparatus itself should be installed in a dust free room. There is also provided a liquid applicator in the form of a drum "38 comprised of a shaft '31 on which are strung a very large number of thin discs 32 of polyethylene film held together between end discs 34 of Lucite by nuts 35 threaded on the shaft at the ends of thedrum. The discs dip into a tank 36 of liquid which will act as a release agent for the carbonate film from the casting wheel '22, as for example; 2,2-aminomethylamino ethanol (NH CH CH NHCH CH OH), which has a boiling point of about C. at a pressure of twelve millimeters of mercury, and the thinness of the film of liquid can be adjusted very minutely by loosening or tightening the Lucite discs on the shaft or by shifting the axle carrying the polyethylene discs 32 away from or toward the surface of the wheel. This shifting may be accomplished by means of a differential screw 38 having a knurled nut 39, the screw being threaded into a lug 4%) extending down from the tank 36 and into a threaded bearing 41 on the base 4-2 of the machine, the tank 36 being suitably. guidedly supported for sliding movement on said base as byits support post 43 sliding between two dovetailed straps 44. The shaft 31 is driven through a motordriven slow speed gearing t5, the gearing being of the variable character to provide further means to vary the speed of the shaft 31 and therefore the quantity of liquid applied by the applicator to the wheel. Above the applicator is a pair of vertically spaced heating lamps 46 to preliminarily heat the polished surface of the wheel. Thus, the amount of liquid remaining on the wheel depends, in part, onthe choice of rotational speed of the wheel and on the heat cast by the lamps 46 onto the wheel. The lamps are intended to leave a small amount of liquid on the wheel to assist in the subsequent peeling to the cast film from off the wheel. The wheel comprises a central drum 47 of iron or the like having a rim 48 of a highly polished metal such as nickel or chrome to afford a smooth highly polished surface, although nickel steel is preferred.

After the film has been cast on the wheel by flow of material from the hopper ltl, either by action of gravity alone, or. by the action of gravity assisted by air pressure, the film passes beneath an open or perforated bottom box 50 which has air fed thereto by pipe 52. Any suitable form of heatenas finned electric resistance rods 53, is embodied within the box and the air in passing over and about the heater is heated, is blown out of the open or perforated bottom of the box and onto the freshly cast film of carbonates. The film by the time it reaches the line of stripping off which approximately is at the position indicated by the reference character 60 has sufiicient of its Volatile solvents driven off so that the film has enough tenacity to be pulled from 01f the drum and deposited onto an endless mesh conveyor belt 62 travelling in the direction of the arrow in Fig. l. The film is stripped oil? the casting wheel by the pull of the conveyor belt, assisted by the action of an air stripper 64, which blows air through a series of aligned nozzles against the underside of the film at about the designated line 60. In practice, the conveyor is of aluminum and has a 14 x 16 mesh (224 holes to the square inch). It is wider than the width of the film and moves at about twice the lineal speed of the surface of the wheel or about three feet perminute. As it moves, it slips beneath the film as it is being stripped from ofi the wheel. The film is thoroughly dried by air which is heated within an oven housing 66, the air being drawn in through the two open ends of the oven by a motor driven pump 68 and discharged at a suitable location. The oven has within it finned electric resistance rods 69, like those within the box 50. The conveyor is made of the mesh material so as to be sure to obtain an undersurface on the film which is free of bumps and unevennesses. Also the mesh structure of the conveyor permits free access of the heated air to both surfaces of the film as it is being transported by the conveyor.

The dried film is wound up on a spool 70 in our conventional fashion, care being taken that there always is sufiicient slack between the conveyor 62 and the spool 70 to avoid rupture of the film.

The specific mounting of the hopper with relationship to the casting wheel and its adjustment relative thereto to obtain a thin film on the wheel is described in the companion application referred to below to which reference may be had for further details. Since the specific form of hopper is no part of this invention no detailed description thereof is here set forth. In general the hopper is constructed of a vertical rear end wall 80, opposite triangular side walls 82, and an inclined front wall 86.-

The wall 86 inclines downwardly and towardthe rear end wall 80 to form a metered orifice or slot at the bottom of the hopper. The hopper is closed at the top by means of a cover plate 88, removably held to the hopper by a pair of wing nuts 90.

The bottom of the inclined wall '86 pivots on the shoe 102 which has portions which ride directly on the polished periphery of the wheel near the lateral edges thereof. The shoe is restrained against movement around with the circumference of the wheel by a pair of parallel reach bars 115 pivoted on a shaft 116 fixed in upright ears on a bracket 120 extending across the surface of the wheel and supported by framework 121 of the machine. Tilting of the hopper about the bottom of wall 86 on shoe 102 to vary the distance of the mouth of the hopper from the periphery of the wheel is effected by rotation of a knurled screw 148 having threads of one pitch threaded into a nut 134 pivotally mounted on a bracket 130 fastened on the wall 86 of the hopper. The screw has threads of a slightly different pitch threaded into a second nut 142 with trunnions pivoted in an arm 138 fixed to the bracket 120. The differential threads on thescrew 148 enable fine tilting adjustments of the hopper.

The subject matter relating to the structure of the hopper is not the invention of the applicants herein and is claimed in the previously referred to companion application, said application having been filed of even date herewith, Serial #783,788, in the name of Alton L. Crosby and Kenneth M. Stoll, assignors to the assignees of the instant application. This application has issued on December 27, 1960, as Patent 2,965,927.

What is claimed is:

l. The method of forming a film with smooth upper and lower surfaces comprising flowing a suspension of solids within a binder dissolved in a volatile liquid onto a support, preliminarily driving off some of the volatile liquid while the film is on said support to partially set the same, removing the film from off the support and while removing the partially set film sliding a planar surface against the under surface of the film to impart thereto a smooth undersurface, and further drying the film.

2. The method of forming a film with smooth upper and lower surfaces comprising flowing a suspension of solids within a binder dissolved in a volatile liquid onto a support, comprising preliminarily wetting the support with an agent to promote separation of a partially set film from the support, preliminarily driving off some of the volatile liquid while the film is on said support, removing the partially set film from off the support and while re- '4 moving the partially set film sliding a planar surface against the under surface of the film to impart thereto a smooth undersurface, and further drying the film.

3. The method of forming a film with smooth upper and lower surfaces comprising flowing a suspension of solids within a binder dissolved in a volatile liquid onto a support, preliminarily driving off some of the volatile liquid while the film is on said support to partially set the same, applying a pulling sliding surface to the under face of said film to both pull the film ofif the support and to smooth the under surface, and applying a drying current of air to said film while pulling and sliding the surface beneath the film.

4. Mechanism for forming a film with smooth upper and lower surfaces comprising a movable support, means for dispensing onto said support a composition comprising a suspension of solids held in a liquefied binder, heating means applied to the film while on said support for preliminarily drying the same sulficiently to apply to set to the film but not sufiiciently to completely dry the same, a conveyor adjacent to the movable support to receive the partially set film, means for driving said conveyor at a lineal speed in excess of the lineal speed of the support to strip the film from the support and to slide beneath the film to smoothen the under face of the same, and means to further dry the film while on said conveyor.

5. Mechanism for forming a film with smooth upper and lower surfaces comprising a movable support, means for dispensing onto said support a composition comprising a suspension of solids held in a liquefied binder, heating means applied to the film while on said support for preliminarily drying the same sufiiciently to apply a set to the film but not sufficiently to completely dry the same, a conveyor adjacent to the movable support to receive the partially set film, means for driving said conveyor at a lineal speed in excess of the lineal speed of the support to strip the film from the support and to slide beneath the film to smoothen the under face of the same, means to further dry the film while on said conveyor, and means for applying to said support an agent which facilitates the stripping of the film therefrom.

6. Mechanism for forming a film with smooth upper and lower surfaces comprising a movable support, means for dispensing onto said support a composition comprising a suspension of solids held in a liquefied binder, heating means applied to the film while on said support for preliminarily drying the same sufliciently to apply a set to the film but not sufiiciently to completely dry the same, a conveyor adjacent to the movable support to receive the partially set film, means for driving said conveyor at a lineal speed in excess of the lineal speed of the support to strip the film from the support and to slide beneath the film to smoothen the same, and means to further dry the film while on said conveyor, said conveyor being ametallic belt of fine mesh.

7. Mechanism for forming a film with smooth upper and lower surfaces comprising a movable support, means for dispensing onto said support a composition comprising a suspension of solids held in a liquefied binder, heating means applied to the film while on said support for preliminarily drying the same sufliciently to apply a set to the film but not completely dry the same, a conveyor adjacent to the movable support to receive the partially set film means for driving said conveyor at a lineal speed in excess of the lineal speed of the support to strip the film from the support and to slide beneath the film to smoothen the same, and means to further dry the film while on said conveyor, said conveyor being a metallic belt of fine mesh and means for directing heated air about the conveyor to cause the heated air to contact the upper surface of the film directly and the lower surface of the film via the interstices in the conveyor. I

8. Mechanism for forming a film with smooth upper and lower surfaces comprising a movable support, means for dispensing onto said support a composition comprising a suspension of solids held in a liquefied binder, heating means applied to the film while on said support for preliminarily drying the same sufliciently to apply a set to the film but not sufficiently to completely dry the same and a conveyor adjacent to the movable support to receive the partially set film, means for driving said conveyor at a lineal speed in excess of the lineal speed of the support to strip the film from the support and to slide References Cited in the file of this patent beneath the film to smoothen the under face of the same. 10 2,841,827

UNITED STATES PATENTS Cooper June 26, 1894 Zocher \Aug. 30, 1932 1 Land et al. July 10, 1945 Lies Jan. 30, 1951 Ziegler et a1. June 12, 1951 Toulmin Nov. 27, 1951 Crownover July 8, 1958 

